The present invention relates to a dropout detection circuit and an optical disc apparatus incorporating the dropout detection circuit.
An optical disc apparatus reproduces a signal recorded on an optical disc by converging a light beam emitted from a light source such as a semiconductor laser on the optical disc rotating at a predetermined rotational speed to irradiate the optical disc. Such an optical disc apparatus is provided with a dropout detection circuit to deal with an event of failure of good signal reproduction due to generation of a blemish and the like on an optical disc, that is, an event of occurrence of a dropout. Various methods have been proposed for dropout detection. Basically, however, the dropout detection includes detecting a change in the envelope of a reproduced RF signal.
<Dropout Detection>
FIG. 10 is a block diagram of a conventional dropout detection circuit, and FIGS. 11A to 11C are illustrations of the operation of the conventional dropout detection circuit.
A RF signal S100 reproduced from an optical disc is first input into an AGC circuit 145, which is provided generally for stabilizing the amplitude level of the input signal against possible overall and local inconsistencies in the reflection from the disc surface. Without the AGC circuit 145, an inconsistency in the reflection from an optical disc will be detected as an envelope change. Therefore, by inputting the RF signal S100 into the AGC circuit 145, a signal S101 output from the AGC circuit 145 has a waveform stabilized in amplitude level as shown in FIG. 11A.
There are provided two envelope detection circuits having different time constants, high-speed envelope detection circuit 147 having a smaller time constant and a low-speed envelope detection circuit 146 having a time constant larger than that of the high-speed envelope detection circuit 147. For example, when the RF signal S100 of which the envelope sharply changes due to a dropout is input into the envelope detection circuits as shown in FIG. 11A, the high-speed envelope detection circuit 147 having a small time constant outputs a signal S102 having a waveform following the sharp change in the envelope of the RF signal S100 as shown in FIG. 11B. On the contrary, the low-speed envelope detection circuit 146 having a large time constant, which fails to follow the sharp change in the envelope of the RF signal S100, outputs a signal having a waveform changing with a fixed time constant.
The signal output from the low-speed envelope detection circuit 146 having a large time constant is input into a level shift circuit 148, where the signal is changed to a signal S103 provided with a desired voltage difference as shown in FIG. 11B. The signal S103 and the signal S102 output from the high-speed envelope detection circuit 147 having a small time constant are input into a comparator 149. The comparator 149 compares the input signals S102 and S103 with each other, to detect the sharp fall in the envelope of the RF signal S100, that is, the dropout. After comparison, the comparator 149 outputs a dropout detection signal S104 as shown in FIG. 11C.
The dropout detection signal S104 output from the comparator 149 is input into a phase compensation circuit for focusing control and a phase compensation circuit for tracking control. These phase compensation circuits normally perform phase compensation of a signal from an A/D converter at a preceding stage and transmit phase-compensated signals to a next stage as control error signals. When the dropout detection signal S104 is in the “H” level, the phase compensation circuits continue holding the control error signals obtained before the change of the level of the dropout detection signal S104 from “L” to “H”. In this way, a false control error signal generated during a dropout is prevented from being transmitted to the next stage, and thus deviation in focusing control and tracking control due to a false control error signal is prevented.
As described above, dropout detection is performed by detecting a change in the envelope of the reproduced RF signal. This indicates that at least information must be pre-recorded on a medium from which reproduction is made. For example, information is previously recorded on reproduction-only media such as CD, CD-ROM and DVD-ROM in the form of pits. Therefore, detection of a dropout is possible during reproduction. On the contrary, recording media such as CD-R, CD-RW, DVD-R, DVD-RW and DVD-RAM include no information in the initial state, and therefore detection of a dropout during recording is not possible. To overcome this problem, in the case of DVD-RAM, recording is temporarily stopped if the recording starts to go off track, and measures such as storing data in another management region are taken. This is however not applicable to media requiring continuous recording such as DVD-R.
With recent commercialization of various types of recording/reproduction apparatuses, requests for dropout detection not only during reproduction but also during recording have increased. However, in the current level of dropout detection, continuous dropout detection at the switching of the state from reproduction to recording or from recording to reproduction is not possible.